Magnetization precession excited by spin transfer effect in a current-driven
spin-torque nano-oscillator (STNO) can be phase-locked to an external
microwave signal having frequency $f_{e}$ close to the frequency $f_{0}$ of
the STNO precession. In previous studies, only the phase-locking of STNO to
a microwave \textit{current} was observed. In this work we studied experimentally STNO
phase-locking to a microwave \textit{magnetic field} having amplitude $h_{e}$ up to 20 Oe rms. We
observed both main ($f_{e}$/$f_{0}$ = 1) and secondary ($f_{e}$/$f_{0}$ = 2)
locking regimes. For sufficiently large driving field $h_{e}$ both regimes of
the STO phase-locking become hysteretic, with the boundaries of the locking
interval dependent on the direction of the sweep of the external frequency
$f_{e}$. The bandwidth of the main ($f_{e}$/$f_{0}$ = 1) locking regime was
larger than the bandwidth of the secondary ($f_{e}$/$f_{0}$ = 2) regime when
the dc bias magnetic field was perpendicular to the microwave field
$h_{e}$. In contrast, for parallel orientations of dc and microwave magnetic
fields, the secondary synchronization regime was more pronounced.

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2010.MAR.L37.2